This document discusses methods for calculating illumination levels. It introduces the lumen method and point method. The lumen method calculates illuminance based on the total lumens of a light source and area of the surface. The point method calculates illuminance based on light intensity and distance. The zonal cavity method is introduced as an application of the lumen method for interior spaces. It includes coefficient of utilization and light loss factors to account for light lost from the source and room characteristics. Correction factors are needed because not all light reaches the surface and light output decreases over time.

1. Chapter 16
Calculations of Illumination

2. Quantity and Quality of Illumination
In lighting design, several tentative selections
and equipment layouts may have to be made
prior to achieving an optimum solution.
Experienced designers can often shorten
this process.
Quantity and quality of illumination are
inseparable.
High-quality lighting compensates for a lack
of quantity.
Poor lighting requires higher lighting
quantity.

3. Basis for Illumination
Illumination calculations are based on the
principle of luminous flux transfer from the
light source or sources to a surface.
Normally, the transfer is through clean air and is assumed
to have no loss.
Calculating Illuminance: (Chapter 14)
Lumen Method: E = F / A
Point Method: E = I / d2
These definitions MUST be modified when applying in
actual lighting design.

4. Basis for Illumination
Why?
Not all luminous flux coming from the light
source can be aimed totally on the surface.
Light flux of lamps will depreciate with age
How?
By including several correction factors or
coefficients
Applicability of the two methods:
Lumen Method: only practical method to use manually if
the light source is large or is diffused
Point Method: almost exclusively for sports lighting
design, especially for outdoor athletic fields.

5. The Zonal Cavity Method
Application of the lumen method to determine
the horizontal illuminance on a working plane
in an interior space.
Introduces two simple modification factors:
Coefficient of Utilization (CU): percentage of total lamp
lumens that can be utilized in the room based on the room
size, configuration, surface reflectance and performance
characteristics of fixtures.
Light loss factor (LLF): accounts for light losses due to
depreciation.
E = (F/A) x CU x LLF

6. The Zonal Cavity Method
Light loss factor (LLF): represents the components of
light loss of a lighting system initially as well as during
normal operations.
These components may be divided into two groups, with
each group containing several factors:
Group 1: Nonrecoverable light loss factors: inherent in
the installed system and cannot be corrected through
routine maintenance.
Voltage Factor Temperature Factor Tilt Factor
Ballast Factor Fixture surface depreciation factor

7. The Zonal Cavity Method
Light loss factor (LLF):
Group 2: Recoverable light loss factors: can be changed
by regular maintenance, such as cleaning relamping, and
painting of room surfaces.
Lamp lumen depreciation factor
Fixture dirt depreciation factor
Room surface depreciation factor
Lamp burnout factor

8. Point Method
Initial Illuminance: (Chp 14)
Eih=(I/d2
) x cos Θ
Eih =I x (cos3
Θ) / h2
Eiv=(I/d2
) x sin Θ
Eiv =I x (cos2
Θ) x (sin Θ) / h2
Maintained Illuminance:
Emh = Eih x LLF
Emv = Eiv x LLF